1. Field of the Invention
The present invention relates to a press forming apparatus and to a press forming method for producing metal separators for fuel cells. In particular, the present invention relates to a press forming technique for producing metal separators for high precision solid polymer electrolyte fuel cells, which facilitates setting a solid polymer electrolyte fuel cell metal separator in a die when a seal is integrally formed with the metal separator, and which yields a good sealing characteristics between separators when separators are stacked in a stack.
2. Description of the Related Art
In solid polymer electrolyte fuel cells, a separator is applied to both sides of a plate-shaped electrode (MEA: Membrane Electrode Assembly) to form a unit having a layered structure, and plural units are stacked to form a fuel cell stack. The MEA is a three-layered structure in which a polymerized electrolytic membrane, which is made of a resin such as an ion-exchange resin, is held by a pair of gas diffusion electrode plates (positive electrode plate (cathode) and negative electrode plate (anode)). The separator is layered so as to contact the gas diffusion electrode plate of the MEA, and gas passages and coolant passages where gas is circulated between the gas diffusion electrode plate and the separator are formed. According to the fuel cell, hydrogen gas as a fuel is provided to the gas passages facing the gas diffusion electrode plate at the negative electrode plate side, and an oxidizing gas such as oxygen or air is provided to the gas passages facing the gas diffusion electrode plate at the positive electrode plate side, and electricity is thereby generated by an electrochemical reaction.
In general, a metal separator used for the above fuel cell is produced by performing press forming, a piercing process, and a trimming process, in that order. The press forming in the above determines a separator structure, and various techniques have been proposed in the past.
A method as the separator forming technique was proposed (see Japanese Unexamined Patent Application Publication No. 2003-142119), in which a polymer film is disposed between a press forming die and a separator material plate in producing a metal separator for fuel cells which has conductive inclusions in a metallographic structure thereof. A method was proposed (see Japanese Unexamined Patent Application Publication No. 2003-77486), in which a base material is formed by press forming, and is subjected to TRD process (Thermo-Reactive Deposition and Diffusion Process), followed by strain elimination process. A method was proposed (see Japanese Unexamined Patent Application Publication No. 2002-75401), in which press forming is performed by using a die having a shape similar to an external shape of a separator for solid polymer electrolyte fuel cells. The separator has a flat portion at each intermediate portion of a protrusion portion and a recess portion, and has a bend portion having a predetermined curvature at each shoulder portion. A method was proposed (see Japanese Unexamined Patent Application Publication No. 2003-36867), in which a separator material made of synthetic resin having an appropriate cross section as a separator is prepared by injection molding synthetic resin, extrusion forming, or press forming. Then, the entire surface of the separator material is subjected to electroless metal plating. A method having the following steps in a press working method in which plural projections are formed on a plate allowing plastic deformation by pressing was proposed (see Japanese Unexamined Patent Application Publication No. 2000-317531). That is, the method has a first step of forming an overhang portion at a portion for forming a desired projection with a first pressing die, and a second step of completing a projection by extruding a center portion with a second pressing die, which is adjacent to a center of the overhang portion, from a circumferential portion of the overhang portion.
FIGS. 1A to 1D are side views showing in time series a die and a separator material used in methods of conventional press forming processes. FIGS. 1E to 1H are plane views showing conditions of the separator materials corresponding to FIGS. 1A to 1D. In FIGS. 1B, 1C, 1F, and 1G each arrow denotes a direction of stress in the separator material. In FIGS. 1F, 1G, and 1H, each short line denotes a crease generated on the separator material. In the conventional techniques, as shown in FIGS. 1A to 1D, a die having upper and lower peripheral portion holding dies 1 and upper and lower forming dies 2 disposed inside the peripheral portion holding die 1 is used. The upper peripheral portion holding die 1 and the upper forming die 2 are fixed with each other. The lower forming die 2 is slidably movable with respect to the lower peripheral portion holding die 1. The forming die 2 has a buffer and passage groove forming portion 2a, and a hole forming portion 2b which is disposed outside the passage groove forming portion 2a. The buffer and passage groove forming portion 2a has a relatively large number of machined portions, and the hole forming portion 2b has a relatively few machined portions. In the above die, a separator material 3 is provided as shown in FIG. 1A, and is held by the upper and the lower peripheral portion holding dies 1 as shown in FIG. 1B. Then, the separator material 3 is subjected to press forming by moving the lower forming die 2 after that as shown in FIGS. 1C and 1D.
In the case in which the above forming apparatus is used, as shown in FIGS. 1B and 1F, when the peripheral portion of the separator material 3 is held by the peripheral portion holding dies 1, the center portion of the separator material of which a buffer and passage groove portion 3a is formed swells downward with respect to the outer portion thereof, which is a hole forming portion 3b, and pulling thereby occurs in the separator material 3 due to stress in a direction shown by the arrow. Next, as shown FIGS. 1C and 1G; the lower forming die 2 is moved upward. In this case, since the buffer and passage groove portion 3a at a center portion of the separator material 3 is worked more than the buffer and passage groove portion 3a positioned thereroutside, the buffer and passage groove portion 3a is first worked by press forming, so that pulling is further generated. As a result, as shown in FIGS. 1D and 1H, when press forming is completed, working strain is formed at several portions of the separator. As a result, creases and warps are formed, the circumference of the separator material is excessively elongated, and groove depth is shallow. Due to this, after a subsequent trimming process is performed, creases, etc., are maintained as they are, and good size accuracy is not thereby obtained at each portion of the separator. Therefore, when a seal is integrally formed with the separator, setting of the separator in a die is difficult. When separators are stacked in a stack, good sealing characteristics between separators cannot be obtained.